The present invention relates to the magnetic resonance arts. It finds particular application in conjunction with localized coils for medical imaging equipment which receive electromagnetic signals from resonating nuclei and will be described with particular reference thereto. It is to be appreciated, however, that the invention may also find utility in other magnetic resonance and radio frequency antenna applications, such as exciting resonance, chemical analysis, well logging, and the like.
Heretofore, various types of coils have been positioned to receive electromagnetic signals for magnetic resonance imaging and spectroscopy, e.g. whole body, body portion, and localized coils. The whole body and portion receiving coils had standard sizes which were selected for readily receiving the patient's whole body or a selected portion. Due to the standardized coil size and variable patient size, a void or empty region was commonly defined between the coil and the portion of the patient to be imaged.
Localized or surface coils were configured from rigid or flexible non-conductive sheets on which conductive loops were mounted. Rigid, flat coils were constructed in a variety of sizes to facilitate positioning adjacent the selected area of the patient to be imaged. When a flat coil was positioned adjacent to a relatively flat area of the patient, the intervening air gap was relatively small.
To receive signals from deeper within the patient, larger diameter loops were utilized. The depth of the coil's region of sensitivity has been adjusted by selecting more complex winding patterns. However, the complex loop arrangements still had the high magnetic energy losses of the single loop.
Another approach was to construct the localized coil from a loop of quarter length 50 ohm coaxial cable with a gap cut into the outer shield. The quarter wave length loop would be connected integrally with an n.lambda./2 transmission line.
One of the problems with the prior art localized coils resided in coupling between the sample or patient and the coil, which coupling degraded the Q or quality factor of the coil. The closer the surface coil was placed to the patient, the more the coupling problem was accentuated. Although the use of quarter wave length coaxial cable with a gap cut into the outer sheathing reduced such coupling, the coaxial cable coils were load sensitive. Changes in the patient or load would affect the Q value of the coil, the tuning, the matching, and the coil performance.
In the present application, localized coils are provided which overcome the above referenced problems and others. In a receive mode, an improved signal-to-noise ratio is achieved.